1. Field of the Invention
The present invention relates to an image forming apparatus such as a printer, a copying apparatus or a facsimile apparatus, and more particularly to an image forming apparatus capable of detecting a foreign object attached to a recording material.
2. Related Background Art
Image forming apparatus is generally used for forming an image on a recording material such as a plain paper (transfer paper), a postcard, a cardboard, an envelope or a plastic sheet for an overhead projector, and is represented for example by a printer, a copying apparatus and a facsimile apparatus utilizing an electrophotographic process.
An image forming apparatus constituting a background technology of the present invention is illustrated in FIG. 9. As shown in FIG. 9, the image forming apparatus is equipped with a photosensitive drum 1, serving as an image bearing member and rotated in a direction indicated by an arrow.
The surface of the photosensitive drum 1 is uniformly charged by a charging roller 2, and is then subjected to be exposed as an image by exposure means 11 such as a laser, whereby a latent image is formed on the photosensitive drum 1. Then the latent image is developed with toner 3 in a developing device 8 having a developing roller 5, into a visible toner image.
The toner image on the photosensitive drum 1 is carried, by the rotation thereof, to a transfer nip portion (transfer portion) N formed by a transfer roller 4 and the photosensitive drum 1.
On the other hand, a recording material P for image recording, such as a transfer paper, is conveyed from a sheet feed unit (sheet cassette) 15 to the transfer nip portion N, through feed roller pair 16 and paired conveying rollers 17, 18. Then the recording material P is subjected to a transfer of the toner image on the photosensitive drum 1, while being conveyed by the transfer roller 4 in the transfer nip portion N.
The recording material P, bearing the transferred toner image, is conveyed to a fixing device 24 constituted of a heating rotary member 76 and a pressurizing rotary member 77 which constitute a fixing nip portion H, and is subjected therein to heat and pressure whereby the toner image is fixed onto the recording material.
As explained above, the image forming apparatus, after forming a toner image on a recording material, heats and pressurizes the recording material, bearing the toner image, in a fixing device, thereby achieving fuse fixation of the toner image onto the recording material. For image fixation, there is widely known a fixing device of contact heating type, with satisfactory thermal efficiency and safety.
As the fixing device, there is principally employed a heat roller type fixing device constituted by contacting a heat roller and a pressure roller. The heat roller is provided with a releasing layer on the surface of a cylindrical metallic core and incorporates a halogen heater inside the cylinder, while the pressure roller is generally formed by forming an elastic layer of heat-resistant rubber on a metal core and forming a releasing layer thereon.
Also for an even higher heating efficiency, a film-type heat fixing device is recently proposed and utilized, employing a fixing film and a ceramic heater instead of the heating roller. The fixing film is constituted of a heat-resistance resinous film of a low heat capacity such as of polyimide (PI) and a releasing layer formed thereon, and, in the fixing nip portion, the recording material is heated by contacting a ceramic heater across the fixing film.
In such fixing device, as the recording material is heated while it is pinched and pressed in the fixing nip portion, an eventual foreign object present on the recording material may cause a damage on the components of the fixing device, depending on a size, a hardness and a shape of such foreign object. Particularly in a film-type heat fixing device, utilizing a thin fixing film of a heat-resistant resin of a thickness of 100 μm or less, there may result not only a surfacial damage but also a phenomenon of hole formation in the film itself (hereinafter called “film break”), eventually leading to a defective fixed image or a termination in the function of device itself.
FIGS. 10 and 11 illustrate an example in which such film break is caused when a recording material, including a staple 85, is conveyed.
Referring to FIG. 10, when a recording material P bearing a staple 85 (particularly in case an end of the staple being exposed on the surface) enters the fixing nip portion H, the staple end pierces the surface of the film 78 and is dragged toward the downstream side to form a hole 78g in the film 78 as shown in FIG. 11, thereby inducing a local fixation failure and an image perturbation in the subsequent fixed images. Also cracks are generated from such hole in the subsequent use, finally leading to a breakage of the film 78.
In this manner, the presence of foreign object on the recording material has not been much experienced in the past and in the ordinary method of use, but, with recently increased consciousness on economical and environmental issues, documents once prepared and used are not discarded but re-used utilizing the unprinted rear side. Such once-used documents include those bound together with a staple or With a paper clip. Therefore, in case of handling a large amount of documents for such re-use, the user may overlook some of such staples or paper clips and may execute printing operation while the paper still bears such staple or paper clip, thereby leading to a trouble of film breakage.
In the prior image forming apparatus, there is proposed, as a countermeasure against such foreign object present in the recording material, to provide an exclusive metal detector utilizing a magnetic sensor or to utilize a magnet for removing such foreign object.
However such metal detector utilizing a magnetic sensor or such magnet leads to a complication and an increased cost of the apparatus.